Double heterojunction diode lasers conventionally consist of an active region layer sandwiched between two relatively thick layers of a material having a bandgap higher than the bandgap of the active region layer material. The active region layer and the sandwiching layers are doped such that one of the heterojunctions is a rectifying junction, with forward bias of the rectifying junction providing carrier injection into the active region layer to provide, upon carrier recombination, the generation of radiation and with the sandwiching layers providing carrier and radiation confinement. If the active region layer of this conventional laser is relatively thick (1.0-2.0 microns) many transverse optical fields (transverse modes) are allowed to propagate within the active region layer. Generally, this multiple transverse mode behavior is undesirable, in part because the resulting output radiation lacks coherency and collimation, and also because the relative power in each mode is uncontrolled.
To obtain transverse mode control (TMC) the active regions of some double heterojunction lasers are made thin, for example, between 0.1 microns and 0.5 microns. This active layer thickness allows only one transverse mode to "fit" into the laser waveguide. These thin active region lasers have the disadvantage that the total power they generate is limited first because of the small active volume of the active region layer available for carrier recombination and radiation production and second because of the thick sandwiching layers required on both sides of the active region layer. Thick sandwiching layers are necessary to confine the light with little loss when the active region layer is thin but these sandwiching layers generally have low heat conductivity which prevents carrier recombination and absorption produced heat from being dissipated readily from the active region layer.
Lasers with thin active region layers also produce an output beam having a greater divergence than the output beam from lasers with a thicker active region layer. By "divergence" it is meant that the beam spreads out as it moves away from the emitting end of the laser. In many applications of semiconductor lasers the emitted beam is directed at a target and it is desirable that the beam impinge upon the target as a spot of controlled area. Therefore, it is desirable that the emitted beam have a minimum of divergence so as to simplify the lens system which may be needed in the optical system between the laser and the target.
One prior art semiconductor double heterojunction diode laser purportedly achieves low emitted beam divergence by making the bandgap energy difference at one heterojunction greater by a substantial amount than the bandgap energy difference at the other heterojunction, with a five to one difference being taught. Specifically, the difference is achieved by varying the composition of the thick sandwiching layers such that one of these layers has a bandgap lower by a substantial amount than the bandgap of the other sandwiching layer. For example, the low bandgap sandwiching layer can be Ga.sub.0.97 Al.sub.0.03 As and the high bandgap sandwiching layer can be Ga.sub.0.9 Al.sub.0.1 As. It is believed that the mismatch in heterojunction bandgap energy difference shifts the transverse mode patterns such that they are assymetrical with respect to the center of the active region layer such that the normalized area of the lowest order mode is greater than the normalized area of the other modes. Since the normalized area of a laser mode is a direct function of the gain seen by that mode, the mismatched energy difference at the heterojunctions will encourage lowest order mode operation and accordingly low beam divergence.
Providing lowest order mode operation and low beam divergence by heterojunction band gap energy difference mismatch has several disadvantages. First, the lower band gap sandwiching layer substantially decreases current confinement. Second, the thick sandwiching layers exhibit poor heat conductivity which prevents carrier recombination and absorption produced heat energy from leaving the active region layer.